2010-02-15_HYDROLOGY - M2007044 - Laserfiche WebLink

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Given the threat that radioactivity from uranium may pose to human health, it is advisable to limit uranium concentrations in streams to the maximum extent practicable. For segments assigned a water supply classification the Commission has adopted a standard of 40 pCi/1 or natural background where higher, for the following reasons: 1. 40 pCi/1 generally reflects background concentrations of uranium that may be found in streams in Colorado and therefore this amount approximates routine human exposure. 2. The statistical risk of human health hazards is small at 40 pCi/l. 3. 40 pCi/1 is an interim level, established now pending the outcome of further studies currently underway. VIII. Water Ouality Standards for Cyanide The Commission acknowledges that total cyanide is to be used in State Discharge Permits until a method is authorized by EPA for measuring free cyanide, even though free cyanide is the parameter of concern. IX. Water Quality Standards for Metals Several parties were concerned about the methods that were employed to digest samples used to determine ambient metal values. The Commission heard testimony that when high suspended solids are present, the two methods of sample digestion could result in very different values, with the"total"method yielding the higher values. But when the suspended solids are low,the two digestion methods will result in similar values, Therefor, the Commission has incorporated data generated by the "total" method when it could be determined that the suspended solids in the water sample were low. The Commission believes that the "total recoverable" or equivalent method should be used as a testing method for determining ambient metal values for streams. This method is a better indicator in determining the amount of metal available to aquatic life in a toxic form, particularly when the amount of suspended solids carried by the stream is high. However, with low suspended solids the two testing methods should yield the same result. Therefore, before incorporating into stream standards data generated by the"total" method it must be verified that there are low suspended solids in the water samples tested. The United States Geological Survey used the "total" method before 1978 and the"total recoverable" after 1978, and that because of this, there might be some inconsistency in the STORET data. The Commission believes that with the proper check on suspended solids, pre- 1976 STORET data can be used to determine ambient stream values. It was suggested by AMAX that since the "total" method is used in monitoring state discharge permits,then the "total" method should be used in setting stream standards. The Commission does not agree. For the reasons already stated,the Commission believes that the "total recoverable " is preferable for assigning water quality standards. And, since most state discharge permits limit suspended solids to 30 mg/l, effluent testing will be similar to the methods underlying stream standards. X. Linka¢e of classifications and Standards The Commission holds that the classifications which it adopts and the standards it assigns to them are linked. Disapproval by EPA of the standards may require reexamination by the Commission of the appropriateness of its original classification. The reason for the linkage is that the Commission recognizes that there is a wide variability in the types of aquatic life in Colorado streams which require different levels of protection. Therefore, the numbers were chosen in some cases on a site specific basis to protect the species existing in that segment. If any reclassification is deemed a downgrading, then it will be based upon the grounds that the original classification was in error. 14